Spur Gears

What is a spur gear?

A spur gear is the simplest and most widely used type of gear. It is a round wheel with straight teeth cut parallel to its axis, designed to mesh with another spur gear to transmit power and motion between two parallel shafts. When one gear turns, its teeth push against the teeth of the other gear, turning it in the opposite direction. By pairing gears with different numbers of teeth, spur gears can reduce or increase speed and torque, making them a key part of gearboxes, drives, and mechanisms of all sizes.

Spur gears are made in a wide range of materials and sizes, from small precision moulded acetal gears used in instruments and light mechanisms, to strong steel gears used in industrial gearboxes and heavy machinery.

A typical spur gear has:

• Teeth - straight-cut teeth around the outside, which mesh with the matching gear
• Face width - the width of the teeth across the gear, which affects its strength
• Hub or boss - an extended centre on many gears that holds the bore and gives room for a set screw, pin, or keyway
• Bore - the hole that fits the shaft, either a pilot bore, a finished bore, or a tapered bore for a locking bush
• Web or body - the section between the hub and the teeth, which may be solid, recessed, or spoked

Spur gears are usually described by:

• Module - the tooth size in metric gears, such as 0.5, 1.0, 1.5, 2.0, or 3.0 module. A larger module means larger, stronger teeth
• Number of teeth - such as 12, 15, 18, 30, or 60 teeth
• Pressure angle - the angle of the tooth profile, usually 20 degrees
• Pitch circle diameter - the effective working diameter, which for metric gears is the module multiplied by the number of teeth
• Outside diameter - the diameter across the tips of the teeth, which for standard metric gears is the module multiplied by the number of teeth plus two
• Bore - the diameter of the shaft hole, such as 2 mm, 3 mm, or 10 mm
• Material - such as acetal, nylon, steel, cast iron, or brass

For example, a 0.5 module moulded acetal spur gear with 18 teeth and a 3 mm bore has a pitch circle diameter of 9 mm and an outside diameter of 10 mm.

Key features:

• Parallel shaft drive - transmits power between shafts that are parallel to each other
• Speed and torque change - different tooth counts reduce or increase speed and torque
• High efficiency - spur gears transmit power with very low losses
• Positive drive - meshing teeth give accurate, slip-free motion
• No axial thrust - straight teeth do not push the gear along the shaft
• Wide choice of materials - quiet, lightweight acetal gears through to strong steel gears
• Wide range of sizes - available in many modules, tooth counts, and bore sizes

In simple terms, a spur gear is a straight-toothed wheel that meshes with another gear to transmit power between parallel shafts, changing speed and torque as needed.

Why are spur gears essential?

Spur gears are essential because almost every machine needs to transmit power and adjust speed or torque. Motors often run too fast for the parts they drive, and spur gears provide a simple, efficient, and reliable way to reduce speed and increase torque. They can also increase speed, reverse the direction of rotation, and link several shafts together in a gear train for accurate timing and positioning.

Their main benefits are:

• High efficiency - very little power is lost through the gears
• Accurate motion - positive drive with no slipping, ideal for timing and positioning
• Speed and torque control - ratios allow speed and torque to be matched to the load
• Simple design - easy to calculate, fit, and replace
• No thrust loads - straight teeth do not push along the shaft, keeping bearing arrangements simple
• Quiet, low-maintenance options - moulded acetal gears run quietly and often need little or no lubrication
• Reliable - long-lasting when correctly set up and lubricated where needed
• Cost-effective - economical to produce and widely available

Spur gears are used across a wide range of industries and applications, including:

• Gearboxes - speed reducers and increasers
• Instruments and electronics - small drives in meters, sensors, and control equipment
• Office and consumer equipment - printers, scanners, appliances, and small mechanisms
• Automation and robotics - drives, actuators, and positioning systems
• Machine tools - lathes, mills, and feed drives
• Packaging and food machinery - drives and timing systems
• Medical and laboratory equipment - quiet, clean, precise drives
• Agricultural machinery - drives, gearboxes, and implements
• Rack and pinion drives - spur gears used as pinions to drive gear racks
• Model engineering, robotics kits, and hobby projects - small gear trains and mechanisms

The main types of spur gears

Spur gears are usually grouped by material, shape, bore type, and measurement system.

By material:

• Moulded acetal spur gears - precision moulded from acetal, a tough engineering plastic also known as POM. They are lightweight, quiet, low friction, and resistant to wear, moisture, and corrosion, and can often run with little or no lubrication. Ideal for small, light-duty drives, instruments, and mechanisms, and commonly available in small modules such as 0.5 module, with a range of tooth counts and bore sizes
• Nylon spur gears - lightweight, quiet, and wear resistant, used for light and medium-duty drives
• Steel spur gears - strong and durable, often made from En8 or similar steels, for general and heavy-duty drives
• Hardened steel spur gears - induction or case hardened for higher loads and longer wear life
• Cast iron spur gears - stable and good at damping vibration, used for larger gears
• Brass spur gears - corrosion resistant and smooth running, used for instruments and light mechanisms
• Stainless steel spur gears - for corrosive, food, and marine environments

By shape:

• Spur gears with a hub - have an extended boss that holds a set screw, pin, or keyway, making them easy to fix to a shaft
• Plain spur gears - flat gears without a hub, used where space is limited or where they will be fixed to another part
• Spoked or recessed spur gears - larger gears with material removed from the web to reduce weight
• Pinions - small spur gears with fewer teeth, often used to drive larger gears or gear racks
• Internal gears - with teeth cut on the inside of a ring, used in planetary gearboxes and compact drives
• Idler gears - placed between two gears to change direction or bridge a gap, without changing the overall ratio

By bore type:

• Pilot bore gears - supplied with a small starter bore that can be machined to suit the shaft, often with a keyway added
• Finished bore gears - supplied ready-bored to a standard shaft size, such as 2 mm or 3 mm on small acetal gears
• Taper bore gears - with a tapered bore to accept a tapered locking bush, allowing larger gears to suit many shaft sizes

By measurement system:

• Metric spur gears - sized by module, the standard for most modern machinery
• Imperial spur gears - sized by diametral pitch, found on older and American machinery. Metric and imperial gears are not interchangeable

How to choose the right spur gears?

Choosing the right spur gears means matching them to the drive, load, space, and environment. Work through these points in order:

• Module - choose a module large enough for the load. Both meshing gears must have the same module
• Pressure angle - make sure both gears have the same pressure angle, usually 20 degrees
• Ratio - choose tooth counts that give the speed and torque change you need. The ratio is the number of teeth on the driven gear divided by the number of teeth on the driving gear
• Centre distance - check the gears suit the distance between the shafts. For standard metric gears, the centre distance is the module multiplied by the total number of teeth on both gears, divided by two
• Number of teeth - avoid very small tooth counts where possible, as small pinions with few teeth can be weaker and wear more quickly
• Torque and speed - check the gears can handle the torque and speed of the application
• Material - choose moulded acetal or nylon for quiet, lightweight, low-maintenance drives, and steel or hardened steel for higher loads and temperatures
• Bore - choose a gear with a bore that matches your shaft, such as 2 mm or 3 mm for small acetal gears, or a pilot bore gear that can be machined to size
• Fixing method - consider how the gear will be fixed to the shaft, such as a press fit, set screw, pin, key, or locking bush
• Space - check the outside diameter, face width, and hub size fit the available space
• Environment - consider moisture, temperature, chemicals, and cleanliness. Acetal gears suit clean, dry, and damp environments, while steel gears suit higher loads and temperatures
• Quality and brand - choose gears from reputable suppliers for accurate tooth profiles and smooth meshing

How to fit spur gears

Fitting spur gears correctly is important for smooth, quiet running:

• Support the shafts rigidly - mount shafts in bearings close to the gears to prevent flexing
• Keep shafts parallel - make sure both shafts are parallel, so the teeth mesh evenly across their full width
• Set the centre distance - position the shafts at the correct centre distance for the gears. Too close causes binding and wear, while too far apart causes noise and weak meshing
• Allow backlash - leave a small amount of free play between the teeth to allow for lubrication and heat expansion. Plastic gears may need slightly more allowance, as they expand more with heat and moisture
• Align the gears - make sure the gear faces line up, so the teeth engage across their full width
• Secure the gears - fix the gears to their shafts with a suitable method, such as a press fit, set screw, pin, key, or locking bush
• Lubricate if needed - apply suitable grease or oil to steel, cast iron, and brass gears. Acetal gears can often run with little or no lubrication
• Check the mesh - turn the gears by hand to check for smooth, even meshing without tight spots

FAQ - All you need to know about spur gears

What is a spur gear?

A spur gear is a round gear with straight teeth that meshes with another gear to transmit power between parallel shafts.

What does module mean?

Module describes the size of the teeth on metric gears. A larger module means larger, stronger teeth. Meshing gears must have the same module.

How do I work out the pitch circle diameter?

For metric gears, multiply the module by the number of teeth. For example, a 0.5 module gear with 18 teeth has a pitch circle diameter of 9 mm.

How do I work out the outside diameter?

For standard metric gears, multiply the module by the number of teeth plus two. For example, a 0.5 module gear with 18 teeth has an outside diameter of 10 mm.

How do I work out the centre distance between two gears?

For standard metric gears, add the number of teeth on both gears, multiply by the module, then divide by two. For example, two 0.5 module gears with 12 and 36 teeth have a centre distance of 12 mm.

How do I work out the gear ratio?

Divide the number of teeth on the driven gear by the number of teeth on the driving gear. For example, a 15 tooth gear driving a 45 tooth gear gives a 3:1 ratio, reducing speed to a third and increasing torque.

What are the benefits of moulded acetal spur gears?

Moulded acetal gears are lightweight, quiet, and resistant to wear, moisture, and corrosion. Their low-friction surface means they can often run with little or no lubrication, making them ideal for small drives, instruments, office equipment, and clean environments where grease is not wanted.

When should I choose steel gears instead of plastic gears?

Choose steel gears for higher loads, higher temperatures, heavy shock loads, or where very long life under heavy use is needed. Plastic gears are best for light to medium loads, quiet running, and low-maintenance drives.

Can I mix plastic and steel gears?

Yes. Running a plastic gear with a steel gear is common and can give quieter running and good wear life, as long as both gears have the same module and pressure angle and the load suits the plastic gear.

Why is correct centre distance important?

If gears are too close, they bind and wear quickly. If they are too far apart, the teeth do not mesh properly, causing noise, vibration, and tooth damage.

What is the difference between module and diametral pitch?

Module is used for metric gears and describes the tooth size in millimetres. Diametral pitch is used for imperial gears and describes the number of teeth per inch of pitch diameter. Gears using different systems will not mesh correctly.

Do spur gears create thrust loads?

No. Because the teeth are straight, spur gears do not push along their shafts, which keeps bearing arrangements simple.

Why are my spur gears noisy or wearing quickly?

Common causes include incorrect centre distance, too little or too much backlash, misaligned or non-parallel shafts, overloading, lack of lubrication on metal gears, or mismatched gears. Check the setup and replace worn gears, ideally as a pair.